Generally, it is necessary to impart a certain degree of tension to a chain, belt or similar power transmission device to prevent noises or unmeshing of teeth (in case of a toothed belt). On the other hand, some measures have to be provided to remove excessive tensioning forces.
For example, in a known hydraulic ball-type check valve tensioner used for a camshaft drive chain, a plunger is biased in a protruding direction by a spring and tends to press the tensioner according to the degree of slackening of the chain. A clearance is formed between the ball and seat of the check valve to permit free flow of fluid therethrough. Therefore, the hydraulic pressure from an external source such as an oil pump or the like flows into a chamber through passages formed in the housing, advancing the plunger easily by the combined efforts of the hydraulic pressure and the spring force.
On the other hand, when the plunger tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict outflow of fluid from the chamber, thereby preventing retraction of the plunger to provide the tensioner with a so-called no-return function.
The "no-return" function means that movements are easy in one direction but difficult in the reverse direction.
However, the above-described tensioner construction fails to maintain a predetermined tension, such as when an engine is idling or at rest with low or no oil pressure. Unless appropriate oil pressure is applied to the chamber or it is filled with sufficient oil, the plunger becomes easily movable in both directions and loses the "no-return" function, producing vibrations and noises due to the failure in maintaining a predetermined degree of tension in the chain.
Accordingly, it is an object of the present invention to provide a tensioner for chain, belt or similar wrapped power transmission devices which can maintain a constant tensioning force with compliance for overtensioning.
It is another object of the present invention to provide a fluidic tensioner which can maintain the tension of a chain or the like even when the fluid pressure to the tensioner is low or zero.